Nighttime power storage systems, decentralized household-use power storage systems based on solar power generation technology, power storage systems for electric vehicles, and so forth have attracted widespread attention in recent years because they help protect the global environment and conserve natural resources by utilizing energy more efficiently.
The most important requirement of these storage systems is that the energy density of the battery used should be high. In an effort to meet this requirement, organizations such as the Lithium Battery Energy Storage Technology Research Association (LIBES) have been actively developing lithium ion batteries as promising candidates for high-energy-density batteries.
The second most important requirement is that the power of these storage systems should be stable. For instance, when combining a high-efficiency engine and an energy storage system (such as in a hybrid electric vehicle), or a fuel cell and an energy storage system (such as in a fuel-cell electric vehicle), if the engine or fuel cell is to operate at maximum efficiency, it is essential that it operates at a constant power, and high-rate power discharge characteristics and/or high-rate charging characteristics are required in an energy storage system in order to accommodate power fluctuations in the load or energy regeneration.
Today, an electric double layer capacitor employing activated carbon in the electrode is commonly used as a high-power energy storage device, and large capacitors whose power densities exceed 2 kW/L have been developed. However, since the energy density thereof is only about 1 to 5 Wh/L, such devices, when used alone, do not lend themselves well to the above-mentioned energy storage systems.
Meanwhile, nickel hydrogen batteries, which are employed today in hybrid electric vehicles, have a high power density of over 2 kW/L and an energy density of about 160 Wh/L. Still, tremendous effort has been poured into research aimed at further enhancing the reliability of the battery by further increasing its energy density and also improving its stability at high temperatures.
In the field of lithium ion batteries, researches have proceeded into increasing power density. For example, lithium ion batteries with high power exceeding 3 kW/L at a DOD of 50% have been developed, but these batteries have no more than 100 Wh/L of energy density and are actually designed to suppress high energy density, which is the most characteristic feature of a lithium ion battery.
Thus, there is a strong demand for a battery that combines high power (at least 2 kW/L) and high energy density (180 Wh/L), but so far no battery that satisfies these technical requirements has been developed.
In order to achieve high energy density and high power density simultaneously in a lithium ion battery, it is necessary to adopt a multi-pronged approach to improve the characteristics of the various cell constituent materials, such as the negative electrode material, positive electrode material, electrolyte and so forth. At present, when negative electrode materials such as carbon materials and graphite-based materials are utilized in lithium ion battery production, the capacity decreases markedly during rapid discharge of, for example, about 5 minutes (current density of 4000 mA/g) as compared to a slower discharge of about 1 hour (current density of 300 mA/g). Therefore, a significant breakthrough is required in order to develop a lithium-based secondary battery that combines high energy density and high power density.
Furthermore, lithium-containing metal oxides used as positive electrode materials of lithium ion batteries (typified by LiCoO2, LiMn2O4, LiNiO2 and so forth) make the battery capacity drop markedly during a high-rate discharge of about 5 minutes as compared to a 1C discharge, similar to the graphite-based materials used for the negative electrode, so once again a significant breakthrough is needed to improve the performance of a lithium-based secondary battery.
Meanwhile, the activated carbon used in the capacitor, a high power device, generally has a specific surface area of at least 1000 m2/g. Even if such capacitor-use activated carbon is doped with lithium ions, the efficiency thereof is extremely low, and the density of an electrode obtained therefrom is also low, thus making it difficult to use such activated carbon in a battery having high power and high capacity.
Therefore, a primary object of the present invention is to provide a lithium-based secondary battery that has high energy density and high power density.